The present invention is in the field of geometrical instruments with a central processing unit for making measurements in a well borehole environment. More specifically, the present invention relates to, measurement apparatuses having a rolling contact borne on by a flexible material being measured, the contact caused to rotate by the relative movement between the contact and the material; and having a CPU system for performing the geometric calculation to yield a corrected length measurement.
Logging operations in oil and gas wells require accurate determination of the location of the logging tool in the borehole of the well. Various strategies have been developed to accomplish the accurate location determination of the logging tool during logging operations. One of these strategies involves monitoring the length of the cable or wireline supporting the logging tool payed into or reeled out of the well bore by the cable draw works. Then the downhole length of the cable is used to determine the location of the logging tool in the borehole during the logging operation. xe2x80x9cCount wheelxe2x80x9d or xe2x80x9ccounter wheelxe2x80x9d systems are an example of a technology using this strategy to monitor the length of cable payed into a well bore from a logging tool draw works.
A specific example of a counter wheel type depth measurement system for wireline is disclosed in Kerr, U.S. Pat. No. 5,351,531. Counter wheel systems similar to that disclosed in Kerr typically rely on the use of at least one specifically calibrated count wheel which is matched precisely to the diameter of the wireline or cable loaded onto the cable drum/reel of the draw works to generate a distance of travel signal for the length of cable payed out or reeled in. However, if the operator of the draw works is required to change the cable supply reel or otherwise alter the size/diameter of the cable or wireline used in the draw works, the counter wheel must also be changed out to provide a counter wheel matched to the specific size/diameter of changed cable. Additionally, counter wheels are precision components and subject to replacement upon excess wear, as wearing of the counter wheel will adversely affect the accuracy of the cable length measurement. The matched counter wheel is required in such systems as Kerr because the distance of travel signal in Kerr relates to the length of the surface of the cable engaged with the count wheel, which mathematically is a function of the radius of the count wheel to its wheel surface. However, the distance of travel signal indicating count wheel surface travel does not relate to the true (or mean) length of the cable passing through the device. To relate to the true length of the cable passing through a count wheel type measuring assembly, the count wheel""s distance of travel signal must be corrected for the diameter of the specific cable engaged in the measuring assembly. Previously in the field, this correction was provided by using a count wheel matched to the size of cable/wireline loaded into the draw works. However, having to match the count wheel to the cable necessitates changing the count wheel whenever the size of the cable is changed.
Although the measurement system of Kerr and related devices may be useful for their intended purpose, it would be beneficial in the field to have an alternative measurement apparatus that did not require changing out the count wheel whenever the diameter of the cable was changed. Additionally, in wireline measurement systems like that of Kerr, where a full loop of the cable is engaged on the count wheel, the count wheel is always in use, causing continuous wear on a component of the system which directly impacts the system""s measurement accuracy. It would be further beneficial to the field to have an alternative measurement apparatus in which the high precision measurement component could be readily disengaged from the cable without otherwise interfering with operation of the draw works.
The present invention is an apparatus useful for determining the length of a cable downhole in a well bore, e.g., a wireline cable used in a hydrocarbon well logging operation. The present downhole cable length measuring apparatus can be practiced with standard wireline cable draw works as are typically used on drilling rigs for well logging operations. The downhole cable length measuring apparatus is disposed between the cable drum or reel of the draw works and the well bore.
The downhole cable length measuring apparatus comprises three major components: a rotational distance measuring assembly; a rotation-to-length (rotation:length) calibration assembly; and a processor unit.
The rotational distance measuring assembly component is disposed in engagement with the wireline cable, and typically receives the cable from a draw works or a wireline service truck. The rotational distance measuring assembly has a rotation sensor which generates a rotational distance signal in response to movement (the paying-out and reeling-in) of the wireline cable.
The rotation:length calibration assembly component also engages the wireline or cable, typically more proximate the well bore relative to the length measuring assembly. However, the rotation:length calibration assembly only intermittently engages the wireline cable, and therefore is subject to substantially less wear than the counter wheel. The rotation:length calibration assembly has a length sensor which generates a length calibration signal in response to the length of the cable in passing through the rotation:length calibration assembly.
The processor component receives and processes the sensor signals generated by the other components of the apparatus to provide at least one processor output signal relating the length of wireline cable passing through the apparatus and downhole in the well bore. The processor unit includes a CPU which accomplishes the processing function of the present apparatus.
It is an object of the present invention to provide a downhole cable length measuring apparatus wherein the cable length measurement can be determined independent of the apparatus"" counter wheel circumference or the diameter of the cable or wireline engaged in the apparatus.